ARTICLES 219 



a faint line is distinguishable corresponding to 39. It is just 

 possible that this is a third isotope. 



At first sight it may seem incredible that chlorine, whose 

 chemical combining weight has been determined more often 

 and with greater accuracy than almost any other element, 

 should not have given evidence of its isotopic nature in the 

 past. But it must be remembered that, in all probability, 

 every one of these determinations has been performed with 

 chlorine originally derived from the sea in which the isotopes, 

 if ever separate, must have been perfectly mixed from the most 

 remote ages. Chlorine from some other source, if such can 

 be found, may well give a different result, as did radio-lead 

 when examined. 



Argon {At. Wt. 39*88). — The spectra obtained from this 

 element show its main constituent to be of mass 40, with 

 second order line 20 and third order line i3'33 (Spectra V and 

 VI). The last is very well placed for measurement, and from 

 it the mass of the singly charged atom is found to be 40-00 

 ± -02. At first this was thought to be the only constituent, 

 but later a faint companion was seen at 36, which further 

 spectra showed to bear a very definite intensity relation to the 

 40 line. This is probably an isotope the presence of which to 

 the extent of about 3 per cent, would account for the fractional 

 atomic weight. 



Hydrogen {At. Wt. i-oo8) and Helium {At. Wt. ygg). — ^The 

 determination of masses so far removed from the reference 

 lines as these offered peculiar difficulties ; but as the lines 

 were expected to approximate to the terms of the progression 

 I, 2, 4, 8, etc., the higher terms of which are known, a special 

 method was adopted by which a 2 to i relation could be 

 tested. From the dynamics of the instrument it can be 

 shown that, if an exposure is made and then the potential 

 applied to the electric plates exactly doubled, and another 

 exposure given, the magnetic field being kept constant, all 

 masses having a 2 to i relation will be brought into coin- 

 cidence on the plate. Such coincidences cannot be detected 

 on the same spectrum photographically ; but if we first add 

 and then subtract a small potential from one of the above 

 potentials, two lines will be obtained which will closely bracket 

 the third. Thus the hydrogen molecule line was found sym- 

 metrically bracketed by a pair of atomic lines (Spectrum VII, 

 a and c), showing that the mass of the atom is exactly half the 

 mass of the molecule. When the same procedure was applied 

 to the helium line and that of the hydrogen molecule, the 

 bracket was no longer symmetrical (Spectrum VII, b), nor was 

 it when the hydrogen molecule was bracketed by two helium 

 lines {d). Both results show in an unmistakable manner that 



